The purpose of the proposed studies is to determine if the expression of Gross virus (GV)-induced antigens in the raidoresistant thymic epithelium of newborn mice affects the ability of these mice as adults to respond immunologically to GV-induced antigens on the surface of cultured, syngeneic leukemia cells. Previous studies have demonstrated that mice inoculated at birth with GV are not capable of rejecting cultured, syngeneic GV-induced tumor cells or of mounting a T cell response against these tumor cells whereas both these events occur in normal mice. In these studies the role of thymic antigen expression in this observed lack of response will be examined in F1 thymic chimeras which have received thymus grafts from normal and/or neonatally GV-infected mice from parental strains. These chimeras will be tested for their ability to reject and to mount a T cell response to parental GV-induced tumor cells. Since, by virtue, of the tropism of the virus and the Fv-1 type of the host, the Gross virus should not replicate in host cells, the results of these studies should reflect the consequence of virus expression only in the infected donor thymus. In addition, these studies will determine if nonresponsiveness to syngeneic GV-induced tumor cells in neonatally GV-infected mice is caused by the generation of suppressor cells or by the clonal elimination of virus antigen reactive T cells.